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Legendre polynomial
noun
Legendre transformation
noun
(mathematics)
Given
a
function
f(x,y,z,...)
which
is
concave
up
with
respect
to
x
(i.e.,
its
second
derivative
with
respect
to
x
is
greater
than
zero),
an
involutive
procedure
for
replacing
x
with
another
variable,
say
p=∂f/∂x
thus
yielding
another
function,
say
F=F(p,y,z,...).
This
new
function
contains
all
of
the
information
of
the
original
f
encoded,
as
it
were,
within
it
so
that
∂F/∂p=x
and
applying
a
similar
transformation
to
F
yields
the
original
f.
The
formula
is:
F(p,y,z,...)=p·x(p)-f(x(p),y,z,...)
where
x
must
be
expressed
as
a
function
of
p.
(Note:
The
concave
upwardness
means
that
∂f/∂x
is
monotonically
increasing,
which
means
that
p
as
a
function
of
x
is
invertible,
so
x
should
be
expressible
as
a
function
of
p.)